1. Field of the Invention
The present invention is generally related to continuous scan film readers and strip digitizer systems and, in particular, to a high-speed, continuous linear film transport system enabling continuous, highly accurate media-based image recognition.
2. Description of the Related Art
Substantial libraries of documents and related graphical image forms of information have been archived over the years on roll film media. In particular, there are a considerable number of large archives of imaged documents stored on roll microfilm. Conversion to a digital form is generally desired to prevent loss of the information due to aging of the film media and to fundamentally improve and ensure permanent access to the documents and information.
Many different roll film transport systems, using either a stop-motion or continuous feed architecture, have been developed over the years. These systems have met with varying degrees of success, depending on the nature of the intended application, when considered based on criteria including accurate image reproduction, total throughput, media wear, ease of operation, and both system and operational cost. Stop-motion systems are typically employed where accurate reproduction and high image resolution are required. Media transport mechanics intermittently decelerate the film media to allow static, frame-by-frame projection and, for digitization, two-dimensional image capture. In general, film loops are required to account for the frame-by-frame deceleration requirement for projection. U.S. Pat. No. 4,022,525 employs uncontrolled loops only presumed to provide sufficient slack to account for the individual frame deceleration and projection times. U.S. Pat. No. 6,120,151 provides an improved system where the individual film media frames are compressed into mechanical cartridges and from which the frames are decelerated for individual projection.
Generally, stop-motion systems are disfavored in many different applications due to not least the mechanical complexity and substantial media wear incurred by such systems. The repeated, high-frequency impulse flexion of the film media is well-recognized to directly limit the useful life of the film media. Additionally, the mechanics required to achieve the high deceleration rates necessary for nominal operating throughput rates are themselves a substantial source of non-linear media skew or weave and vibration, directly impacting the accuracy of reproduction.
Continuous transport systems are generally preferred where high-throughput is desired and, in many cases, to avoid the problems associated with stop-motion systems. These systems typically employ constant tensioning systems to align and control the speed of the film media through a film gate. A high-speed digital line scanner is typically positioned transverse to the film gate and configured with a line scan orientation perpendicular to the transport direction of the film media. As the film continuously passes through the film gate, digital scan lines are aggregated and images of the film media contents extracted.
By the nature of the line scanner and related electronics, potentially high throughput rates can be achieved by continuous transport systems. As film media speeds are increased, however, the quality of the image produced by conventional continuous transport systems is progressively compromised. Image accuracy is lost predominantly due to the increasing impact of media transport speed variations and associated randomly varying skew imposed on the media as the media passes through the film gate. Conventional attempts to alleviate these problems have been made by augmenting continuous film media feed systems with a perforation detector, as shown in U.S. Pat. No. 6,091,446, and improved speed control electronics, as shown in U.S. Pat. No. 6,169,571. Use of a perforation detector allows associated electronics to measure the weave movement of the media, at least as between successive sprocket holes, and thereby permit a corresponding correction in the physical positioning of the line scanner. The speed of the film media can be better maintained by actively monitoring, using suitable electronics, the speed and phase relationship of both the sprocket drive and a tensioning capstan positioned at either end of the film gate.
Increasing speed also tends to impose increasing tensional loads on the film media, both intended to better maintain media positioning and unintended as a consequence of proportionally increased speed variation. Additionally, increased speed also increases both mechanical and media wear. Although generally less than the impulse loads and wear imposed by stop-motion systems, increased speed factors directly into an increased risk of loss of information and throughput should the film media be damaged or break. Short of catastrophic media failures, higher speeds conventionally result in increased routine maintenance requirements, increased unscheduled repairs, and decreased overall media life.
A somewhat related throughput problem is that the typical complexity of the film transport path leads to difficulties in loading and aligning new film rolls. Often, the film media must be threaded through and carefully aligned over a complex set of rollers and bails. The direct result is an effective loss of throughput due to the significant time taken to load new rolls and to reload rolls in the event of misalignment errors. The complexity of conventional film transport paths also leads to increased operator costs, particularly due to the need for increased system training and to continually monitor system operations.
Consequently, the practical maximum throughput speed of conventional continuous film media transport systems has been rather limited. Consequently, there is a clear need for a high-speed linear film transport system enabling continuous, highly accurate image recognition.